Self-illuminating devices are known which utilize a glass tube coated on the inside with a phosphor and filled with a radioactive gas such as tritium or krypton 85. In the manufacture of very small light sources of this self-illuminating type, it has been the practice to provide an elongated tube which is coated inside with a luminophor, evacuated, filled with the radioactive gas, and sealed at the ends. The tube is then subdivided by cutting the tube into sections by melting the glass, fusing the glass in the process to seal the ends of the sections and prevent escape of the radioactive gas. Laser beams have been used to heat the glass tube in the subdividing process. U.S. Pat. Nos. 3,706,543 and 3,817,733 describe arrangements incorporating laser beams for subdividing radioactive gas-filled tubes.
Various problems have been encountered in applying the techniques described in the prior art. In the prior art, the practice has been to hold the tube in two spaced chucks which rotate the tube in the laser beam so that the beam effectively scans the whole perimeter of the tube during the melting process. In practice, it has been found difficult to rotate the two chucks exactly in unison while the cut is completed. Any slight deviation from perfectly uniform rotation tends to twist the tube slightly or bend it, which may cause fractures. The larger the cross-sectional dimensions of the tube the more difficulty is experienced in maintaining perfect alignment and rotation of the tube during the cutting process.
Another problem encountered is that the tube is not easily cut at more than one point at a time, since each segment must be held and rotated in exact alignment with all the other segments during the cutting process so that no stress is placed on the tube. As a result, it has been the practice to only cut one segment at a time, advancing the tube between cuts to provide successive cuts along the length of the tube. As each cut takes place, the total volume of the radioactive gas is reduced and therefore the pressure is increased. As each section is cut off, the pressure in the remaining portion of the tube increases by an incremental amount. Thus there is a difference in gas pressure between the first segment cut off the tube and the last segment cut from the tube. This results in a nonuniformity of brightness in the successive tube segments. This increase in pressure also requires an adjustment of the external pressure with each successive cut to maintain the pressure differential necessary to achieve proper collapse and fusion of the tube ends as each new segment is cut.